Recombinant activin a precursor protein

ABSTRACT

Provided is a recombinant activin A precursor protein that has human activin A activity. Also provided is a novel method for producing human activin A. The present invention pertains to a recombinant activin A precursor protein that includes a human mature activin A sequence and a non-human activin A prodomain, wherein said recombinant activin A precursor protein is not a naturally occurring activin A precursor protein. The present invention also pertains to a method for producing a human activin A using the aforesaid recombinant activin A precursor protein.

This application is a Continuation of, and claims priority under 35 U.S.C. § 120 to, International Application No. PCT/JP2019/011357, filed Mar. 19, 2019, and claims priority therethrough under 35 U.S.C. § 119 to Japanese Patent Application No. 2018-053947, filed Mar. 22, 2018, the entireties of which are incorporated by reference herein. Also, the Sequence Listing filed electronically herewith is hereby incorporated by reference (File name: 2020-09-18T_US-618_Seq_List; File size: 39 KB; Date recorded: Sep. 18, 2020).

TECHNICAL FIELD

The present invention relates to a recombinant activin A precursor protein, a method for producing human activin A using the same, and the like.

BACKGROUND ART

Proteins belonging to the TGF-β family contribute to the generation of living organisms, differentiation, adjustment of cell proliferation, and the like, and are also important as differentiating factors for undifferentiated cells. However, since these proteins have complicated disulfide bonds in the molecules, it is often the case that when these proteins are produced using Escherichia coli and the like, the proteins are not correctly folded and fail to reproduce the conformations, thus having no activity.

Activin A, which belongs to the TGF-β family, is a protein present as a homodimer in which two polypeptide chains are linked by multiple disulfide bonds. Activin A is expressed as a precursor (also referred to as pro-activin A), and the precursor has a mature activin A sequence and an activin A prodomain. It is known that as a precursor (pro-activin A), activin A has no activity, and by cleaving the prodomain portion, the mature activin A sequence is released and forms active activin A having a predetermined conformation (Xuelu Wang et al., Nature Communications, 2016, 7:12052). In addition, activin A is known to induce iPS cells to differentiate into definitive endoderm.

Although there have been attempts to produce human activin A by utilizing microorganisms such as Escherichia coli as hosts, human activin A thus produced is often not correctly folded, and fails to reproduce the conformation, thus having no activity. For this reason, it is difficult to produce human activin A having activity by utilizing microorganisms such as Escherichia coli as hosts.

A method for refolding the conformation of human activin A having no activity into the conformation of human activin A having activity has been known (International Publication No. 97/23638). However, since it is necessary to conduct the refolding method separately after producing human activin A utilizing microorganisms such as Escherichia coli as hosts, this method cannot be said to be simple and convenient.

In addition, as described above, it is known that as the precursor, activin A has no activity, and by cleaving the prodomain portion, the mature activin A sequence is released and forms active activin A. An activin A precursor protein having activity while having the prodomain portion has not been reported to date.

SUMMARY OF INVENTION

An aspect of the present invention is to provide a novel method for producing human activin A.

In addition, another aspect of the present invention is to provide a recombinant activin A precursor protein for use in the novel method for producing human activin A.

Furthermore, still another aspect of the present invention is to provide a recombinant activin A precursor protein having human activin A activity.

The present inventors attempted to produce human activin A by using plant cells. However, the accumulation of target human activin A was not confirmed when merely expressing a gene encoding only a human mature activin A sequence forming human activin A having activity in plant cells. Even for animals that express activin A, it has been originally impossible to express only the active type (mature activin sequence), and it was found to be significantly difficult to express only the active type (mature activin sequence).

As a result of earnest studies, the present inventors newly found that it is possible to produce human activin A having activity by expressing, in plant cells, not a gene encoding only a human mature activin A sequence, but a gene encoding a recombinant activin A precursor protein including a human mature activin A sequence and a non-human activin A prodomain to generate the recombinant activin A precursor protein, and subsequently cleaving the human mature activin A sequence from the recombinant activin A precursor protein.

In addition, the present inventors newly found that the recombinant activin A precursor protein including the human mature activin A sequence and the non-human activin A prodomain has human activin A activity by itself.

The present invention may include, for example, the following aspects:

It is an aspect of the present invention to provide a recombinant activin A precursor protein comprising a human mature activin A sequence; and a non-human activin A prodomain, wherein the recombinant activin A precursor protein does not naturally occur.

It is a further aspect of the present invention to provide the recombinant activin A precursor protein as described above, wherein the non-human activin A prodomain is naturally derived from an amphibian, a reptile, or a fish.

It is a further aspect of the present invention to provide the recombinant activin A precursor protein as described above, wherein the non-human activin A prodomain is naturally derived from a fish.

It is a further aspect of the present invention to provide the recombinant activin A precursor protein as described above, wherein the activin A prodomain comprises a sequence having 90% or more identity with a sequence shown in SEQ ID NO: 1.

It is a further aspect of the present invention to provide the recombinant activin A precursor protein as described above, wherein the sequence having 90% or more identity with the sequence shown in SEQ ID NO: 1 is located at positions corresponding to positions 187 to 196 of a sequence shown in SEQ ID NO: 2.

It is a further aspect of the present invention to provide the recombinant activin A precursor protein as described above, wherein the non-human activin A prodomain has 90% or more identity with a sequence shown in SEQ ID NO: 2.

It is a further aspect of the present invention to provide the recombinant activin A precursor protein as described above, wherein the non-human activin A prodomain has 90% or more identity with a sequence represented by SEQ ID NO: 3.

It is a further aspect of the present invention to provide the recombinant activin A precursor protein as described above, wherein the non-human activin A prodomain has 90% or more identity with a sequence shown in SEQ ID NO: 4.

It is a further aspect of the present invention to provide the recombinant activin A precursor protein as described above, wherein the non-human activin A prodomain has 90% or more identity with a sequence shown in SEQ ID NO: 5.

It is a further aspect of the present invention to provide the recombinant activin A precursor protein as described above, wherein the non-human activin A prodomain has 90% or more identity with a sequence shown in SEQ ID NO: 6.

It is a further aspect of the present invention to provide the recombinant activin A precursor protein as described above comprising a sequence specifically cleavable by a protease between the human mature activin A sequence and the non-human activin A prodomain.

It is a further aspect of the present invention to provide a polynucleotide encoding the recombinant activin A precursor protein as described above.

It is a further aspect of the present invention to provide a recombinant vector comprising the polynucleotide as described above.

It is a further aspect of the present invention to provide a transformant comprising the polynucleotide as described above or the recombinant vector as described above.

It is a further aspect of the present invention to provide a method for producing the recombinant activin A precursor protein as described above comprising a step of, in a plant cell into which the polynucleotide as described above or the recombinant vector as described above has been introduced, or in a plant comprising the plant cell, expressing the polynucleotide.

It is a further aspect of the present invention to provide a method for producing human activin A comprising the steps of in a plant cell into which a polynucleotide encoding a recombinant activin A precursor protein as described above or a recombinant vector comprising the polynucleotide has been introduced, or in a plant comprising the plant cell, expressing the polynucleotide to generate the recombinant activin A precursor protein; and treating the recombinant activin A precursor protein with the protease.

In one embodiment, a recombinant activin A precursor protein having human activin A activity is provided. “Human activin A activity” as used herein may mean the property of activating the Smad protein, which is a signaling factor, and/or the property of inducing iPS cells to differentiate into definitive endoderm.

In one embodiment, a novel method for producing human activin A can be provided. One aspect of this producing method may be simpler than the conventional methods. One aspect of this producing method may be more economical than the conventional methods. Moreover, one aspect of this producing method may have a higher production efficiency than the conventional methods.

The producing method as described herein makes it possible to produce human activin A that maintains human activin A activity, unlike the producing methods utilizing microorganisms such as Escherichia coli.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates photographs showing results of the western blotting with an anti-HA antibody. Lanes 1 and 2 show the results using extracts obtained by expressing a recombinant activin A precursor protein including a green sea turtle-derived prodomain in Nicotiana benthamiana and extracting proteins at the 4th day and the 5th day, respectively. Lanes 3 and 4 show the results using extracts obtained by expressing a recombinant activin A precursor protein including a zebrafish-derived prodomain in Nicotiana benthamiana and extracting proteins at the 4th day and the 5th day, respectively.

FIG. 2 illustrates a photograph showing results of the western blotting with an anti-activin A antibody. Lane 1 show a Co column-purified furinΔC protease; lane 2 shows a recombinant activin A precursor protein including a Co column-purified zebrafish-derived prodomain; lane 3 shows reducing conditions for a mixture of the Co column-purified furinΔC protease and the recombinant activin A precursor protein including the zebrafish-derived prodomain; and lane 4 shows non-reducing conditions for the mixture of the Co column-purified furinΔC protease and the recombinant activin A precursor protein including the zebrafish-derived prodomain.

FIG. 3 illustrates a graph showing results of measuring the activin A bioactivity. The horizontal axis indicates, in order from the left, an experimental plot not containing activin A as a negative control (activin A 0); an experimental plot containing activin A produced by R&D systems as a positive control (activin A 100 ng/ml); an experimental plot containing activin A produced by the method as described herein in an amount of 1/1000 of the medium (the recombinant activin A precursor protein including the zebrafish-derived prodomain+furinΔC protease 1/1000); and an experimental plot containing activin A produced by the method as described herein in an amount of 1/100 of the medium (the recombinant activin A precursor protein including the zebrafish-derived prodomain+furinΔC protease 1/100). The vertical axis indicates the amount of Cerberus 1 (pM).

FIG. 4 illustrates a graph showing evaluation of activity of the recombinant activin A precursor protein by the Smad reporter assay. FIG. 4 shows, in order from the left, activin A produced by R&D systems (R&D ActA); a recombinant activin A precursor protein including an African clawed frog-derived prodomain (Xeno-Pro ActA); a recombinant activin A precursor protein including an African clawed frog-derived prodomain+furinΔC (Xeno-Pro ActA/Furin); a recombinant activin A precursor protein including a green sea turtle-derived prodomain (Umi-Pro ActA); a recombinant activin A precursor protein including a green sea turtle-derived prodomain+furinΔC (Umi-Pro ActA/Furin); a recombinant activin A precursor protein including a zebrafish-derived prodomain (Zeb-Pro ActA); and a sample to which no activin A was added (Mock). The horizontal axis in R&D ActA indicates the concentration (ng/mL), and the horizontal axis in those other than R&D ActA and Mock indicates the dilution ratio of the purified protein extract to the medium, or the dilution ratio of the partially purified mixed solution obtained by mixing the purified furinΔC and the recombinant activin A precursor protein to the medium. The vertical axis indicates a relative Smad activity.

FIG. 5 illustrates a graph showing evaluation of activity of the recombinant activin A precursor protein by the Smad reporter assay. FIG. 5 shows, in order from the left, activin A produced by R&D systems (R&D ActA); a recombinant activin A precursor protein including a zebrafish-derived prodomain (Zeb-Pro ActA); a recombinant activin A precursor protein including a channel catfish-derived prodomain (Amenama-Pro ActA); a recombinant activin A precursor protein including a coelacanth-derived prodomain (Latime-Pro ActA); and a sample to which no activin A was added (Mock). The horizontal axis in R&D ActA indicates the concentration (ng/mL), and the horizontal axis in those other than R&D ActA and Mock indicates the dilution ratio of the purified protein extract to the medium. The vertical axis indicates a relative Smad activity.

DESCRIPTION OF EMBODIMENTS

Recombinant Activin A Precursor Protein

An embodiment as described herein relates to a recombinant activin A precursor protein (hereinafter, sometimes referred to as the “recombinant activin A precursor protein of the present invention”) including a human mature activin A sequence and a non-human activin A prodomain, wherein the recombinant activin A precursor protein is not a naturally occurring activin A precursor protein. In one aspect, the recombinant activin A precursor protein including a human mature activin A sequence and a non-human activin A prodomain may be used in a novel method for producing human activin A. In addition, it is known that the precursor protein of activin A including a non-human activin A prodomain has no human activin A activity; however, in one aspect, it is possible that the recombinant activin A precursor protein including: a human mature activin A sequence; and a non-human activin A prodomain has human activin A activity.

“Human mature activin A sequence” as used herein means the C-terminus amino acid sequence of activin A which has been cleaved from the precursor protein of activin A (pro-activin A) expressed in a human living organism and which actually has activity. The “human mature activin A sequence” is a sequence made of 116 amino acids at positions 304 to 419 in a sequence shown in SEQ ID NO: 8, which will be described herein.

“Non-human activin A prodomain” as used herein means an inactive N-terminus amino acid sequence included in a precursor protein of activin A (pro-activin A) expressed in a living organism other than the human by the living organism, and is an amino acid sequence of a part excluding the C-terminus amino acid sequence of activin A which actually has activity and a protease recognition sequence adjacent to the N-terminus side of the C-terminus amino acid sequence.

In one embodiment, the non-human activin A prodomain is an activin A prodomain of, that is, naturally occurring or derived from, a bird, an amphibian, a reptile, a fish, or a mammal other than a human, and that derived from an amphibian, a reptile, or a fish is a particular example. Although examples of the non-human activin A prodomain will be given below, the non-human activin A prodomain is not limited to only the examples given below.

The activin A prodomain of a fish can include a sequence having an amino acid sequence identity of 90% or more with an amino acid sequence of DTRRSGWHTL (SEQ ID NO: 1), and another example includes a sequence having an amino acid sequence identity of 90% or more with the sequence shown in SEQ ID NO: 1 at positions corresponding to positions 187 to 196 of a sequence shown in SEQ ID NO: 2, which will be described herein, but is not limited to these.

For example, the activin A prodomain of a fish includes those selected from the amino acid sequences of the activin A prodomains derived from or native to the following living organisms: zebrafish (Danio rerio), channel catfish (Ictalurus punctatus), coelacanth (Latimeria chalumnae), Sinocyclocheilus rhinocerous, Sinocyclocheilus anshuiensis, Sinocyclocheilus grahami, common carp (Cyprinus carpio), goldfish (Carassius auratus), grass carp (Ctenopharyngodon idellus), Pundamilia nyererei, Mexican tetra (Astyanax mexicanus), Maylandia zebra, Japanese rice fish (Oryzias latipes), platy (Xiphophorus maculatus), guppy (Poecilia reticulata), sailfin molly (Poecilia latipinna), amazon molly (Poecilia formosa), Atlantic molly (Poecilia mexicana), barramundi (Lates calcarifer), Atlantic salmon (Salmo salar), northern pike (Esox lucius), Asian arowana (Scleropages formosus), Maylandia Zebra, haplochromis (Haplochromis burutoni), yellow croaker (Larimichthys crocea), mangrove killifish (Kryptolebias marmoratus), spiny chromis (Acanthochromis polyacanthus), yellowtail amberjack (Seriola lalandi dorsalis), spotted gar (Lepisosteus oculatus), silver salmon (Oncorhynchus kisutch), Atlantic herring (Clupea harengus), walking catfish (Clavas batrachus), piranha (Pygocentrus nattereri), rainbow trout (Oncorhynchus mykiss), ocellaris clownfish (Amphiprion ocellaris), bicolor damselfish (Stegastes partitus), greater amberjack (Seriola dumerili), Hong Kong catfish (Clavas gariepinus), tilapia (Oreochromis niloticus), annual killifish (Austrofundulus limnaeus), mummichog (Fundulus heterochtus), southern platyfish (Xiphophorus maculatus), Neolamprologus brichardi, wrasse (Labrus bergyha), and Asian swamp eel (Monopterus albus).

The activin A prodomain of a fish may be selected from the amino acid sequences of the activin A prodomains derived from or native to living organisms belonging to Cypriniformes, Silunformes, or Coelacanthiformes. In addition, the activin A prodomain of a fish may be selected from the amino acid sequences of the activin A prodomains derived from living organisms belonging to Cyprinidae, ktaluridae, or Latimeriidae, or may be selected from the amino acid sequences of the activin A prodomains derived from or native to living organisms belonging to Danio, Ictalurus, or Latimeria.

Throughout this specification, the expression “derived from” can be used interchangeably with “native to”.

The non-human activin A prodomain may be one having an amino acid sequence identity of 90% or more, 95% or more, or 98% or more with the amino acid sequence of the activin A prodomain of a fish, and is not limited to the above-described examples.

The non-human activin A prodomain has an amino acid sequence identity of 90% or more, 95% or more, or 98% or more with the amino acid sequence (SEQ ID NO: 2) of the activin A prodomain derived from zebrafish. The non-human activin A prodomain can include the amino acid sequence of the activin A prodomain derived from zebrafish. The non-human activin A prodomain can be made of the amino acid sequence of the activin A prodomain derived from zebrafish.

The amino acid sequence identity herein can be calculated using a publicly-known analysis tool, and for example, can be calculated using the identity algorithm BLAST (Basic local alignment search tool) of the National Center for Biotechnology Information (NCBI). In addition, for the calculation of the amino acid sequence identity, parameters of the default (initial setting) in the analysis tool may be used.

The non-human activin A prodomain has an amino acid sequence identity of 90% or more, 95% or more, or 98% or more with the amino acid sequence (SEQ ID NO: 3) of the activin A prodomain derived from channel catfish. The non-human activin A prodomain can include the amino acid sequence of the activin A prodomain derived from channel catfish. The non-human activin A prodomain can be made of the amino acid sequence of the activin A prodomain derived from channel catfish.

The non-human activin A prodomain has an amino acid sequence identity of 90% or more, 95% or more, or 98% or more with the amino acid sequence (SEQ ID NO: 4) of the activin A prodomain derived from coelacanth. The non-human activin A prodomain can include the amino acid sequence of the activin A prodomain derived from coelacanth. The non-human activin A prodomain can be made of the amino acid sequence of the activin A prodomain derived from coelacanth.

For example, the activin A prodomain of an amphibian includes those selected from the amino acid sequences of the activin A prodomains derived from the following living organism: African clawed frog (Xenopus laevis), Japanese fire belly newt (Cynops pyrrhogaster), western clawed frog (Xenopus tropicalis), Nanorana parkeri, common coqui (Eleutherodactylus coqui), and American bullfrog (Lithobates catesbeiana).

The activin A prodomain of an amphibian may be selected from the amino acid sequences of the activin A prodomains derived from living organisms belonging to Anura. In addition, the activin A prodomain of an amphibian may be selected from amino acid sequences of the activin A prodomains derived from living organisms belonging to Pipidae, or may be selected from amino acid sequences of the activin A prodomains derived from living organisms belonging to Xenopus.

The non-human activin A prodomain may be one having an amino acid sequence identity of 90% or more, 95% or more, or 98% or more with the amino acid sequence of the activin A prodomain of an amphibian, and is not limited to the above-described examples.

The non-human activin A prodomain has an amino acid sequence identity of 90% or more, 95% or more, or 98% or more with the amino acid sequence (SEQ ID NO: 5) of the activin A prodomain derived from African clawed frog. The non-human activin A prodomain can include the amino acid sequence of the activin A prodomain derived from African clawed frog. The non-human activin A prodomain can be made of the amino acid sequence of the activin A prodomain derived from African clawed frog.

For example, the activin A prodomain of a reptile includes those selected from amino acid sequence of activin A prodomain derived from the following living organisms: green sea turtle (Chelonia mydas), Indian python (Python molurus), green anole (Anolis carolinensis), central bearded dragon (Pogona vitticeps), Japanese gecko (Gekko japonicus), king cobra (Ophiophagus hannah), garter snake (Thamnophis sirtalis), brown spotted pit viper (Protobothrops mucrosquamatus), painted turtle (Chrysemys picta), Chinese alligator (Alligator sinensis), Indian gavial (Gavialis gangeticus), and American alligator (Alligator mississippiensis).

The activin A prodomain of a reptile may be selected from the amino acid sequences of the activin A prodomains derived from living organisms belonging to Testudines. In addition, the activin A prodomain of a reptile may be selected from the amino acid sequences of the activin A prodomains derived from living organisms belonging to Cheloniidae, or may be selected from the amino acid sequences of the activin A prodomains derived from living organisms belonging to Chelonia.

The non-human activin A prodomain may be one having an amino acid sequence identity of 90% or more, 95% or more, or 98% or more with the amino acid sequence of the activin A prodomain of a reptile, and is not limited to the above-described examples.

The non-human activin A prodomain has an amino acid sequence identity of 90% or more, 95% or more, or 98% or more with the amino acid sequence (SEQ ID NO: 6) of the activin A prodomain derived from green sea turtle. The non-human activin A prodomain can include the amino acid sequence of the activin A prodomain derived from green sea turtle. The non-human activin A prodomain can be made of the amino acid sequence of the activin A prodomain derived from green sea turtle.

The phrase “not a naturally occurring activin A precursor protein” or “does not naturally occur” as used herein means that the recombinant activin A precursor protein is different in amino acid sequence from the precursor proteins of activin A (pro-activin A) produced by the expression of genes of natural living organisms whose genes are not modified. In other words, the recombinant activin A precursor protein including a human mature activin A sequence and a non-human activin A prodomain as described herein is different in amino acid sequence from the precursor proteins of activin A produced by the expression of genes of natural living organisms whose genes are not modified.

For example, some mature activin A sequences in primates such as monkeys have the same sequence as the mature activin A sequence of the human. Hence, a activin A precursor protein including the mature activin A sequence of a monkey and the activin A prodomain of the monkey is deemed to be the same as the activin A precursor protein including the mature activin A sequence of the human and the activin A prodomain of the monkey, and the phrase “not a naturally occurring activin A precursor protein” or “does not naturally occur” as used herein is intended to mean that such an activin A precursor protein is excluded from the scope of the present invention.

In the recombinant activin A precursor protein including a human mature activin A sequence and a non-human activin A prodomain, the human mature activin A sequence can be located closer to the C-terminus than the non-human activin A prodomain. In addition, the recombinant activin A precursor protein can include a sequence specifically cleavable by a protease between the human mature activin A sequence and the non-human activin A prodomain. The sequence specifically cleavable by a protease can be adjacent to the human mature activin A sequence and the non-human activin A prodomain and can be adjacent to the C-terminus side of the non-human activin A prodomain and to the N-terminus side of the human mature activin A sequence.

The sequence specifically cleavable by a protease includes a furin recognition sequence, a TEV protease recognition sequence, an HRV 3C protease recognition sequence, a thrombin recognition sequence, a factor Xa recognition sequence, or an enterokinase recognition sequence, and the like, but the sequence specifically cleavable by a protease is not limited to these. The sequence specifically cleavable by a protease can be one that can be cleaved on the C-terminus side of the sequence, and also can be a furin recognition sequence. The furin recognition sequence is a sequence that generally has arginine at the C-terminus, or has lysine at the C-terminus in some cases, and in which the C-terminus side can be cleaved by furin. For example, a standard furin recognition sequence can be expressed by an amino acid sequence represented by R-X-(K/R)-R (X is any amino acid), but the furin recognition sequence is not limited to this.

The recombinant activin A precursor protein may include a desired tag bound in a genetically engineered manner, for example, the His tag, the HA tag, the FLAG tag, the Myc tag, or the like. As the tag, a tag that does not affect the conformation of the activin A precursor protein can be used, and a tag formed of 5 to 15 amino-acid residues is another example. In the case where the recombinant activin A precursor protein includes a tag, the tag can be located on the N-terminus side of the non-human activin A prodomain.

Polynucleotide that Encodes Recombinant Pro-Activin A Protein

One embodiment as described herein relates to a polynucleotide encoding the recombinant activin A precursor protein. The polynucleotide can be synthesized by those skilled in the art based on the common technical knowledge.

The polynucleotide encoding the recombinant activin A precursor protein may include a nucleic acid sequence encoding any signal sequence bounded to the N-terminus side in a genetically engineered manner. The signal sequence can be an endoplasmic reticulum insertion signal sequence, and an endoplasmic reticulum insertion signal sequence known by those skilled in the art such as one derived from thale cress may be used.

When the recombinant activin A precursor protein includes a tag on the N-terminus side of the non-human activin A prodomain, the polynucleotide encoding the recombinant activin A precursor protein can include a nucleic acid sequence encoding the signal sequence the side closer to the N-terminus than the nucleic acid sequence encoding the tag.

Recombinant Vector

One embodiment as described herein relates to a recombinant vector including a polynucleotide encoding the recombinant activin A precursor protein as described herein. The usable vector may be one that can functionally insert the polynucleotide and can carry the different nucleic acid fragment into a host cell to allow the fragment to be replicated or expressed in the host cell, and includes, for example, plasmids, phages, cosmids, and viruses. The vector can be appropriately selected by those skilled in the art depending on the type of an assumed host cell.

The host cell includes, for example, plant cells, insect cells, fungus cells (yeast), prokaryotic cells (Escherichia coli), animal cells (CHO). Among these, since plant cells are assumed as the host cell, the vector can be an expression vector for plant cells, such as pRI 201 DNA series (TAKARA BIO Inc.), but is not limited to these. An appropriate vector may be selected and used depending on the type of a plant cell.

Transformant

One embodiment as described herein relates to a transformant including a polynucleotide encoding the recombinant activin A precursor protein, or a transformant including a recombinant vector including a polynucleotide encoding the recombinant activin A precursor protein.

“Transformant” as used herein means a host or host cell in which a different nucleic acid fragment has been introduced. The nucleic acid fragment for transformation, that is, the polynucleotide encoding the recombinant activin A precursor protein may be incorporated into the genome of the host or host cell or may not be incorporated. For example, the nucleic acid fragment for transformation may be maintained in an episome or a plasmid. The gene may be appropriately introduced by those skilled in the art based on the technical common knowledge. For example, the transformant can be a plant cell or a plant, and as a method for introducing a gene into a plant cell or a plant, the agroinfiltration method, the plant virus vector method, and the like are known.

Method for Producing Recombinant Pro-Activin A Protein

One embodiment as described herein is a method for producing the recombinant activin A precursor protein. The method for producing a recombinant activin A precursor protein includes a step of, in a plant cell into which the polynucleotide encoding the recombinant activin A precursor protein or a recombinant vector including the polynucleotide has been introduced, or in a plant including the plant cell, expressing a polynucleotide. This allows the plant cell or the plant to produce the target recombinant activin A precursor protein.

In the method for producing a recombinant activin A precursor protein, the expression of the polynucleotide may be transient or stable in a plant cell or a plant. The culture conditions for the plant cell may be selected by those skilled in the art as appropriate based on the common technical knowledge in accordance with the type of the plant cell chosen. For example, the culture medium and the conditions such as the temperature, the humidity, and the concentration of carbon dioxide, and the like of the environment where the culture is to be conducted are not particularly limited as long as the culture medium and the conditions are suitable for each cell, and may be selected from a conventionally known culture medium and conditions. The cultivating conditions for a plant can also be selected by those skilled in the art as appropriate based on the common technical knowledge in accordance with the type of the plant chosen.

The plant used for the method for producing a recombinant activin A precursor protein is not particularly limited, but can be a plant that can be infected with agrobacterium. For example, the plant may be a dicot or may be a monocot. The dicot includes Solanaceae plants such as tobacco, tomato, and potato, Brassicaceae plants such as thale cress, rapeseed, broccoli, and cabbage, Asteraceae plants such as crown daisy and lettuce, Rosaceae plants such as strawberry, Japanese apricot, apple, and peach, Fabaceae plants such as soybean, peanut, and mung bean, Apiaceae plants such as carrot and celery, Cucurbitaceae plants such as melon, watermelon, and cucumber, Convolvulaceae plants such as Japanese morning glory and sweet potato, Chenopodiaceae plants such as spinach, quinoa, and sugar beet, and the like. The monocot includes Poaceae plants such as rice, wheat, barley, and corn, Liliaceae plants such as garlic, onion, and Welsh onion, and the like.

The plant used for the method for producing a recombinant activin A precursor protein can be a Solanaceae plant, and tobacco is another example. Tobacco includes, for example, Nicotiana benthamiana, Nicotiana tabacum, Nicotiana rustica, Nicotiana tomentosifonnis, Nicotiana sylvestris, and the like, and Nicotiana benthamiana is a particular example.

The plant cell used for the method for producing a recombinant activin A precursor protein is also not particularly limited, and cells of the above-described plants may be used.

In the method for producing a recombinant activin A precursor protein, when the recombinant activin A precursor protein includes a sequence specifically cleavable by a protease between the human mature activin A sequence and the non-human activin A prodomain, the sequence specifically cleavable by a protease is one that is not cleaved by a protease present inside the plant cell to be used as the host cell.

The method for producing a recombinant activin A precursor protein may further include a step of extracting, from the plant cell or the plant, a recombinant activin A precursor protein generated in the plant cell or the plant. For example, the protein can be extracted from the plant cell by breaking the tissue using something like a mixer or a mortar with water or a buffer. Such an extracting step can be conducted by those skilled in the art as appropriate based on the common technical knowledge.

The method for producing a recombinant activin A precursor protein may further include a step of purifying the extract containing the recombinant activin A precursor protein extracted from the plant cell or the plant. For example, it is possible to add an epitope tag to the recombinant protein in advance, and purify the recombinant protein by a purifying method according to the added epitope. The epitope tag includes the histidine tag, the HA tag, the GST tag, the MBP tag, the MYC tag, the DYKDDDDK tag, the streptavidin tag, and the like. Such a purifying step can be conducted by those skilled in the art as appropriate based on the common technical knowledge.

Method for Producing Human Activin A

One embodiment as described herein is a method for producing human activin A (hereinafter, sometimes referred to as the “method for producing human activin A of the present invention”), and human activin A produced by this producing method may have human activin A activity.

The method for producing human activin A includes at least the following two steps:

1. In a plant cell into which a polynucleotide encoding the recombinant activin A precursor protein or a recombinant vector including the polynucleotide has been introduced, or in a plant including the plant cell, expressing the polynucleotide to generate the recombinant activin A precursor protein. Here, the recombinant activin A precursor protein includes a human mature activin A sequence and a non-human activin A prodomain and also includes a sequence specifically cleavable by a protease between the human mature activin A sequence and the non-human activin A prodomain; and

2. Treating the recombinant activin A precursor protein with the protease.

In the method for producing human activin A, the recombinant activin A precursor protein is of an aspect that includes a human mature activin A sequence and a non-human activin A prodomain and also includes a sequence specifically cleavable by a protease between the human mature activin A sequence and the non-human activin A prodomain, among the above-described recombinant activin A precursor protein. The sequence specifically cleavable by a protease can be adjacent to the human mature activin A sequence and the non-human activin A prodomain, and can be adjacent to the C-terminus side of the non-human activin A prodomain and the N-terminus side of the human mature activin A sequence.

The sequence specifically cleavable by a protease includes a furin recognition sequence, a TEV protease recognition sequence, an HRV 3C protease recognition sequence, a thrombin recognition sequence, a factor Xa recognition sequence, or an enterokinase recognition sequence, and the like, but the sequence specifically cleavable by a protease is not limited to these. The sequence specifically cleavable by a protease can be one that can be cleaved on the C-terminus side of the sequence, and can be a furin recognition sequence. The furin recognition sequence is a sequence that generally has arginine at the C-terminus (has lysine at the C-terminus in some cases), and in which the C-terminus side can be cleaved by furin. For example, a standard furin recognition sequence can be expressed by an amino acid sequence represented by R-X-(K/R)-R (X is any amino acid), but the furin recognition sequence is not limited to this.

In the method for producing human activin A, the expression of the polynucleotide may be transient or stable in a plant cell or a plant. The culture conditions for the plant cell may be selected by those skilled in the art as appropriate based on the common technical knowledge in accordance with the type of the plant cell used. For example, the culture medium and the conditions such as the temperature, the humidity, and the concentration of carbon dioxide, and the like of the environment where the culture is to be conducted are not particularly limited as long as the culture medium and the conditions are suitable for each cell, and may be selected from a conventionally known culture medium and conditions. The cultivating conditions for a plant can also be appropriately selected by those skilled in the art based on the common technical knowledge in accordance with the type of the plant used.

The plant used for the method for producing human activin A is not particularly limited, but can be a plant that can be infected with agrobacterium. For example, the plant may be a dicot or may be a monocot. The dicot includes Solanaceae plants such as tobacco, tomato, and potato, Brassicaceae plants such as thale cress, rapeseed, broccoli, and cabbage, Asteraceae plants such as crown daisy and lettuce, Rosaceae plants such as strawberry, Japanese apricot, apple, and peach, Fabaceae plants such as soybean, peanut, and mung bean, plants such as carrot and celery, Cucurbitaceae plants such as melon, watermelon, and cucumber, Convolvulaceae plants such as Japanese morning glory and sweet potato, Chenopodiaceae plants such as spinach, quinoa, and sugar beet, and the like. The monocot includes Poaceae plants such as rice, wheat, barley, and corn, Liliaceae plants such as garlic, onion, and Welsh onion, and the like.

The plant used for the method for producing a recombinant activin A precursor protein can be a Solanaceae plant, or tobacco. Tobacco includes, for example, Nicotiana benthamiana, Nicotiana tabacum, Nicotiana rustica, Nicotiana tomentosifonnis, Nicotiana sylvestris, and the like, and Nicotiana benthamiana is a particular example.

The plant cell used for the method for producing a recombinant activin A precursor protein is also not particularly limited, and cells of the above-described plants may be used.

In the method for producing human activin A, a human mature activin A sequence is cleaved by treating the recombinant activin A precursor protein generated in the plant cell or in the plant with the protease. The human mature activin A sequence itself forms human activin A, and human activin A produced by the method for producing human activin A in this manner can maintain its activity.

The protease that can be used in the method for producing human activin A includes the furin protease, the TEV protease, the HRV 3C protease, the thrombin, the factor Xa, or the enterokinase, and the like, but is not limited to these. The protease can be one that cleaves the C-terminus of the recognition sequence, and can be the furin protease. Note that although the furin protease is of lipid-raft type and naturally an insoluble protease, there is a report that the furin protease was converted into secreted soluble type by deleting the lipid-raft region using an insect cell (Bravo, D. a., Gleason, J. B., Sanchez, R I., Roth, R. a. & Fuller, R. S. Accurate and efficient cleavage of the human insulin proreceptor by the human proprotein-processing protease furin. Characterization and kinetic parameters using the purified, secreted soluble protease expressed by a recombinant baculovirus. J. Biol. Chem. 269, 25830-25837 (1994), the contents of which are incorporated herein by reference), and it is possible to use a furin protease obtained by deleting the lipid-raft region present at the C-terminus of such furin protease in the method for producing human activin A.

The treatment with the protease may be conducted in a plant cell or a plant in which the recombinant activin A precursor protein has been generated. In this case, the protease may be a protease that is spontaneously present in a plant cell or in a plant, or a protease that is not spontaneously present in a plant cell or in a plant and may be expressed in a plant cell or in a plant by transformation using gene transfer. Such transfer of a gene into a plant cell or a plant can be appropriately conducted by those skilled in the art based on the common technical knowledge, and can be conducted by a method such as the agroinfiltration method or the plant virus vector method.

In addition, the treatment with the protease may be conducted outside a plant cell or a plant in which the recombinant activin A precursor protein has been generated. In this case, the protease may be a commercially-available protease or a protease prepared separately from the recombinant activin A precursor protein. For example, a furin protease obtained by deleting the lipid-raft region present at the C-terminus of a furin protease results in a secreted soluble type, and it is possible to react the furin protease with the recombinant activin A precursor protein outside a plant cell or a plant by preparing this furin protease separately from the recombinant activin A precursor protein and using the furin protease.

The method for producing human activin A may further include a step of extracting, from the plant cell or the plant, a recombinant activin A precursor protein generated in the plant cell or the plant. For example, the protein can be extracted from the plant cell by breaking the tissue using something like a mixer or a mortar with water or a buffer. Such an extracting step can be conducted by those skilled in the art as appropriate based on the common technical knowledge. This extracting step is conducted before the step of treating the recombinant activin A precursor protein with the protease.

The method for producing human activin A may further include a step of purifying the extract containing the recombinant activin A precursor protein extracted from the plant cell or the plant. For example, it is possible to add an epitope tag to the recombinant protein in advance, and purify the recombinant protein by a purifying method according to the added epitope. The epitope tag includes the histidine tag, the HA tag, the GST tag, the MBP tag, the MYC tag, the DYKDDDDK tag, or the streptavidin tag, and the like. Such a purifying step can be appropriately conducted by those skilled in the art based on the common technical knowledge. This purifying step is conducted before the step of treating the recombinant activin A precursor protein with the protease.

When the step of treating the recombinant activin A precursor protein with the protease has been conducted in a plant cell or in a plant, the method for producing human activin A may further include a step of extracting, from the plant cell or the plant, human activin A generated in the plant cell or the plant after the treating step. For example, the protein can be extracted from the plant cell by breaking the tissue using something like a mixer or a mortar with water or a buffer. Such an extracting step can be appropriately conducted by those skilled in the art based on the common technical knowledge.

The method for producing human activin A may further include a step of purifying the extract containing the human activin A extracted from the plant cell or the plant. For example, it is possible to add an epitope tag to the recombinant protein in advance, and purify the recombinant protein by a purifying method according to the added epitope. The epitope tag includes the histidine tag, the HA tag, the GST tag, the MBP tag, the MYC tag, the DYKDDDDK tag, or the streptavidin tag, and the like. Such a purifying step can be appropriately conducted by those skilled in the art based on the common technical knowledge.

EXAMPLES

Hereinafter, the present invention will be described in further detail with reference to specific Examples. However, the scope of the present invention is not limited to these Examples.

Materials and Method

1. Plant and its Cultivation Method

As the plant, tobacco (cultivar: Nicotiana benthamiana) was chosen. A cultivation soil was prepared by mixing Kumiai Nippi Engei Baido No. 1 and vermiculite in 1:1, and the cultivation soil thus prepared was put into a plug tray. Tobacco seeds were sowed on the plug tray, were cultivated for about 4 weeks, and were then transplanted into a No. 3 vinyl pot. For agroinfiltration, the plant in the second to third week after the transplantation in the vinyl pot was used. The cultivation was conducted at a light intensity of about 70 μmol m⁻² s⁻¹ before the agroinfiltration (in the lower stage of the Biotron B room), and at a light intensity of about 150 μmol m⁻² s⁻¹ after the agroinfiltration (in the upper stage of the Biotron B room).

2. Construction of Plasmid Vector and Artificial Gene Synthesis

Various insert DNAs, which were artificial genes, were inserted between the Ndel site immediately below the AtADH 5′ UTR and the Sall site further downstream of the AtADH 5′ UTR using the pRI201-AN vector (TAKARA BIO Inc.). The Gibson Assembly (Registered Trademark) Master Mix was used for the insertion of the insert DNAs.

The artificial genes were synthesized utilizing the GeneArt (Registered Trademark) Strings™ DNA Fragments service of Thermo Fisher Scientific. The synthesized genes were optimized in conformity with the frequency of codon occurrence of Nicotiana benthamiana using the GeneArt (Registered Trademark) GeneOptimizer (Registered Trademark) provided by Thermo Fisher Scientific.

The chosen artificial genes are shown in Table 1.

TABLE 1 Description of Sequence SEQ ID NO: (ABC-SP)-(His tag)-(HA tag)- SEQ ID NOs: 7, 8 (zebrafish Pro-domain)-(Furin)- (human activin A) (ABC-SP)-(His tag)-(HA tag)- SEQ ID NOs: 9, 10 (channel catfish Pro-domain)- (Furin)-(human activin A) (ABC-SP)-(His tag)-(HA tag)- SEQ ID NOs: 11, 12 (coelacanth Pro-domain)-(Furin)- (human activin A) (ABC-SP)-(His tag)-(HA tag)- SEQ ID NOs: 13, 14 (African clawed frog Pro-domain)- (Furin)-(human activin A) (ABC-SP)-(His tag)-(green sea SEQ ID NOs: 15, 16 turtle Pro-domain)-(Furin)- (human activin A) ABC-SP: endoplasmic reticulum insertion signal peptide derived from Arabidopsis basic chitinase Zebrafish Pro-domain: the activin A prodomain derived from zebrafish Channel catfish Pro-domain: the activin A prodomain derived from channel catfish Coelacanth Pro-domain: the activin A prodomain derived from coelacanth African clawed frog Pro-domain: the activin A prodomain derived from African clawed frog Green sea turtle Pro-domain: the activin A prodomain derived from green sea turtle Furin: the furin recognition sequence Human activin A: the human mature activin A sequence

3. Line and Transformation of Escherichia coli and Agrobacterium

The transformation was conducted using NEB (Registered Trademark) 10-beta strain (NEB) as Escherichia coli and LBA4404 strain (TAKARA BIO Inc.) as agrobacterium in accordance with the protocols attached to the products. First, each plasmid vector was increased in Escherichia coli and was corrected, and subsequently each plasmid vector was introduced into agrobacterium. For the culture of both Escherichia coli and agrobacterium, an LB medium was used. Escherichia coli was cultured at 37° C. and agrobacterium was cultured at 28° C.

4. Agroinfiltration

Agrobacterium holding each transgene was precultured in 3 ml of the LB medium at 28° C. overnight. The agrobacterium suspension thus precultured was 10-fold diluted in the LB medium (1 ml of the agrosolution per 9 ml of the LB), and acetosyringone was added such that the final concentration became 100 μM, followed by main culture for 4 hours. The suspension subjected to the main culture was centrifuged at 4000 g for 5 minutes to collect bacterial cells. The pellets thus collected were suspended such that O.D.600=0.5 in a 10 mM IVIES (pH 5.5)+10 mM MgCl₂ solution, and acetosyringone was added such that the final concentration became 200 μM. The bacterial culture prepared in this manner was allowed to stand at room temperature 25° C. for 1 hour to 3 hours. Thereafter, a 1 ml needleless syringe was used. The syringe was brought into contact with the lower side of the tobacco leaf, then the upper side was pinched with fingers to cause the pressurized state, and the bacterial culture was injected into the leaf (agroinfiltration). After the agroinfiltration, cultivation was conducted at about 150 μmol m⁻² s⁻¹ (in the upper stage of the Biotron B room), and sampling was conducted between the 4th day and the 7th day after the inoculation.

5. Extraction of Protein

After the leaf subjected to the agroinfiltration was collected and the fresh weight was measured, a refrigerate extraction buffer was added in an amount 3 times the fresh weight, and the leaf in the raw or frozen at −80° C. was pulverized with a mortar. As the extraction buffer, a buffer obtained by adding NaCl to D-PBS such that the final concentration became 0.3 to 1 M, further adding Tween-20 in 0.5% to 1%, and adjusting pH to 7.4 was used. For use in metal affinity purification, imidazole was added such that the final concentration became 5 to 15 mM. In addition, commercially-available xTractor buffer (TAKARABIO Inc.) was also used as a buffer dedicated for the metal affinity purification.

6. Metal Affinity Purification

The protein extract was centrifuged with CR22G III centrifuge (Hitachi, Ltd.) at 18,000 g at 4° C. for 15 minutes to cause fragments of cells to precipitate, and the supernatant was recovered. The supernatant was transferred into himac 50 ml culture tube (Hitach, Ltd.) and centrifuged at 32,200 g at 4° C. for 30 to 60 minutes to cause finer fragments to precipitate, and the supernatant was recovered. An appropriate amount of TALON (Registered Trademark) Metal Affinity Resin (TAKARA BIO Inc.) or His60 Ni Superflow Resin (TAKARA BIO Inc.) was added to the supernatant, followed by shaking at 4° C. for 20 minutes or more to allow the Resin to adsorb the protein. Thereafter, the solution containing the Resin was transferred into a gravity column to recover the flow-through. Next, washing was conducted with a Washbuffer (D-PBS+final concentration 0.3 M NaCl, 15 mM imidazole, pH 7.4) in an amount 5 times the volume of the Resin. The elution was conducted by adding an elution buffer for Co resin (D-PBS+final concentration 0.3 M NaCl, 150 mM imidazole, 1 mM CaCl₂), pH 7.4) or an elution buffer for Ni resin (D-PBS+final concentration 0.3 M NaCl, 0.5 M imidazole, 1 mM CaCl₂), pH 8.0) in an amount twice the volume of the Resin to recover the eluate.

7. Western Blotting

The sample at the time of protein electrophoresis was heated at 100° C. for 3 minutes after a sample buffer solution (for SDS-PAGE, 6-time concentrated, containing a reducing agent) pH 6.0 (Nacalai Tesque, Inc.) was added. DTT or mercaptoethanol was used as a reducing agent. The sample for non-reduction was prepared by adding NuPAGE (Registered Trademark) 4×LDS Sample Buffer and treating at 70° C. for 10 minutes. As the polyacrylamide gel, e⋅PAGEL 15% (ATTO) was used to conduct electrophoresis. Thereafter, semi-dry blotting device PoweredBlot Ace (ATTO) was used to transfer the protein to the PVDF membrane. An antibody to the target protein was used as the primary antibody and an HRP antibody to each host animal was used as the secondary antibody. As the anti-activin antibody, a goat-derived polyclonal antibody (R&D systems, product code AF338) was used. As the anti-HA antibody, a rat monoclonal antibody Anti-HA High Affinity (Roche) was used. Detection was conducted by LAS-3000(Fujifilm) using ECL™ Prime Western Blotting Detection Reagent of GE Healthcare.

The results of detection with the anti-HA antibody are illustrated in FIG. 1. Lanes 1 and 2 show the results using extracts obtained by expressing a recombinant activin A precursor protein including a green sea turtle-derived prodomain in Nicotiana benthamiana and extracting proteins at the 4th day and the 5th day, respectively. Lanes 3 and 4 show the results using extracts obtained by expressing a recombinant activin A precursor protein including a zebrafish-derived prodomain in Nicotiana benthamiana and extracting proteins at the 4th day and the 5th day, respectively.

As shown in FIG. 1, a band was detected near an estimated molecular weight (about 44 kDa) of a monomer pro-activin A in the reduced state and a band was detected near an estimated molecular weight (about 88 kDa) of a dimer pro-activin A in the non-reduced state. These suggested that the produced proteins were present in the state where the disulfide bond was able to be cleaved by the reducing agent and were folded in a state similar to the natural state.

The results of detection with the anti-activin A antibody are shown in FIG. 2. Lane 1 shows only the result obtained by deleting the lipid-raft region present at the C-terminus of the furin protease and adding an His tag to synthesize a soluble furin protease (furinΔC protease), followed by the Co column purification. Lane 2 shows only a recombinant activin A precursor protein including a Co column-purified zebrafish-derived prodomain. Lane 3 shows the reducing conditions for a mixture of the Co column-purified furinΔC protease and the recombinant activin A precursor protein including the zebrafish-derived prodomain. Lane 4 shows the non-reducing conditions for the mixture of the Co column-purified furinΔC protease and the recombinant activin A precursor protein including the zebrafish-derived prodomain. The furinΔC protease was obtained, using a plant cell, by synthesizing a soluble furin protease formed by deleting a lipid-raft region present at the C-terminus of a furin protease to convert the furin protease into the secreted soluble type while adding an His tag, as described in Bravo et al. (“Accurate and efficient cleavage of the human insulin proreceptor by the human proprotein-processing protease furin. Characterization and kinetic parameters using the purified, secreted soluble protease expressed by a recombinant baculovirus.” J. Biol. Chem. 269, 25830-25837 (1994)) Although the Bravo et al. uses an insect cell, the synthesis can be made in the same manner using a plant cell. The furinΔC protease was synthesized and purified separately from the recombinant activin A precursor protein including the zebrafish-derived prodomain. In this way, the possibility that as a result of separation of a prodomain and a mature sequence, which is an active domain, due to the cleavage of the recombinant activin A precursor protein including the zebrafish-derived prodomain by the furin protease, the mature sequence was decomposed by an endogenous protease in the plant was avoided.

The furinΔC protease and the recombinant activin A precursor protein including the zebrafish-derived prodomain, which were purified separately, were mixed in a concentration ratio of 7:5 and treated at 30° C. for 30 minutes. As a result, most of the prodomains were cleaved and a band, which was considered to be active under the reducing conditions, was detected near the estimated molecular weight (about 12 kDa). In addition, under the non-reducing conditions, a thin band was detected near the estimated molecular weight (about 24 kDa) of a dimer, because of the poor recognition of the antibody, or other reasons.

8. Measurement of Bioactivity of Activin A

Since the band was detected near about 12 kDa, which was the estimated molecular weight of the active human activin A by cleaving the recombinant activin A precursor protein including the zebrafish-derived prodomain with the furinΔC protease, the bioactivity of the activin A based on the differentiation from the iPS cells to the definitive endoderm (DE) as an index was confirmed as follows.

Human iPS cells (1231A3 strain) maintained and cultured in an AK01 medium were sowed into a 96 well plate coated with iMatrix at 4×104 cells/well. After 24 hours, the medium was replaced with a DE differentiation medium (the activin A produced by the method as described herein or a commercial product, 2% B27 supplement, 3 μM CHIR99021) to initiate differentiation induction. The next day, the medium was replaced with a medium having the same composition and excluding CHIR99021, and thereafter, the medium was replaced every day. The evaluation was made using a commercial product of R&D systems as control, and the amount of Cerberus 1 in the culture supernatant was measured as an index of the DE differentiation. Note that Cerberus 1 is a secretory protein that expresses specifically to the DE stage, and activin A is known to be essential for the differentiation from iPS cells to DE.

Activin A produced by the method as described herein was evaluated using a partially purified mixed solution obtained by mixing furinΔC purified through the Co column and the recombinant activin A precursor protein including the zebrafish-derived prodomain. Since the recombinant activin A precursor protein synthesized in a plant by the method as described herein and purified through the Co column was in a partially purified state and the concentration was unclear, the partially purified mixed solution was added each in an amount of 1/1000 or 1/100 of the medium and used. The supernatant was recovered at the 5th day after the differentiation induction, and the amount of Cerberus 1 in the culture supernatant was quantified by the ELISA method using ES/iPS Differentiation Monitoring Kit (Dojindo, #ES01).

The results of measuring the activin A bioactivity are shown in FIG. 3. The horizontal axis indicates, in order from the left, an experimental plot not containing activin A as a negative control (activin A 0); an experimental plot containing activin A produced by R&D systems as a positive control (activin A 100 ng/ml); an experimental plot containing activin A produced by the method as described herein in an amount of 1/1000 of the medium (the recombinant activin A precursor protein including the zebrafish-derived prodomain+furinΔC 1/1000); and an experimental plot containing activin A produced by the method as described herein in an amount of 1/100 of the medium (the recombinant activin A precursor protein including the zebrafish-derived prodomain+furinΔC 1/100). The vertical axis indicates the amount of Cerberus 1 (pM).

As shown in FIG. 3, in the activity evaluation using DE differentiation as the index, concentration-dependent activity was observed in the human activin A generated by furin protease cleavage, produced using a plant by the method as described herein. In addition, the intensity of the activity was approximately 85% of the R&D 100 ng/mL with the addition in an amount of 1/100.

9. Evaluation of Activity by Smad Reporter Assay

The intensity of the Smad signal activated by activin A was quantified using Cignal Smad reporter assay (QIAGEN) using the human activin A generated by the recombinant activin A precursor protein and the furin protease cleavage, produced by the method as described herein.

First, to 25 μL of the Opti-MEM medium, 0.6 μL of the Attractene reagent (QIAGEN) was added, followed by incubation at room temperature for 5 minutes. To the above-mentioned mixed solution, a mixed solution of 25 μL of the Opti-MEM medium and 1 μL of the Smad reporter vector (QIAGEN) was added, followed by incubation at room temperature for 20 minutes. This mixed solution was added to the Collagen I-coated 96-well plate (IWAKI), and HEK293E cells at 40,000 cell/well were sowed and incubated in a 5% CO₂ incubator at 37° C. overnight to conduct transfection of the Smad reporter vector.

After the transfection was completed, all the culture supernatant was removed.

An Opti-MEM medium (starving medium) containing 0.5% FBS, 1% NEAA, and penicillin-streptomycin was added in an amount of 75 μL, followed by culturing at 37° C. for 4 hours to starve the cells.

The human activin A or the recombinant activin A precursor protein produced by the method as described herein or a starving medium to which activin A produced by R&D systems was added such that the concentration is twice the final concentration of the control was added in an amount of 75 μL to cells, followed by culturing in a 5% CO₂ incubator at 37° C. overnight to stimulate the cells. The human activin A produced by the method as described herein was evaluated using a partially purified mixed solution obtained by mixing furinΔC purified through the Co column and the recombinant activin A precursor protein including the African clawed frog or green sea turtle-derived prodomain in the same manner as in the above-described 8, and was used while being diluted at rates described in FIG. 4. The recombinant activin A precursor proteins synthesized in the plant by the method as described herein were as generated as described above, the recombinant activin A precursor protein including the zebrafish-derived prodomains used the protein extracts purified through the Ni column, and the other recombinant activin A precursor proteins used the protein extracts purified through the Co column, and these were used while being diluted at rates as described in FIGS. 4 and 5. For the concentration of the recombinant activin A precursor protein in each extract, the concentration of the recombinant activin A precursor protein including the African clawed frog-derived prodomain was 362 ng/μL, the concentration of the recombinant activin A precursor protein including the green sea turtle-derived prodomain was 63 ng/μL, the concentration of the recombinant activin A precursor protein including the zebrafish-derived prodomain was 332 ng/μL, the concentration of the recombinant activin A precursor protein including the channel catfish-derived prodomain was 178 ng/μL, and the concentration of the recombinant activin A precursor protein including the coelacanth-derived prodomain was 114 ng/μL. The concentrations were obtained by the Bradford method after the protein purification.

The Dual-Luciferase Reporter Assay System (Promega) was used for the reporter assay. First, 100 μL of the medium supernatant was removed, 50 μL of the Glo reagent was added, and cells were dissolved at room temperature for 10 minutes. The activation of the Smad pathway was quantified by detecting the light emission of Firefly luciferase emitted from the cell solution using a plate reader. Subsequently, 50 μL of a Glo & Stop reagent solution was added, and the light emission of Renilla luciferase, which is an internal standard, was detected after 10 minutes to quantify the number of cells. In accordance with the Smad activity=(the light emission intensity of Firefly luciferase)/(the light emission intensity of Renilla luciferase), the Smad activity of each sample was quantified. The relative value of the Smad activity for the sample (Mock) to which no activin A was added was obtained as Relative Smad activity.

The results are shown in FIGS. 4 and 5. FIG. 4 shows, in order from the left, the activin A produced by R&D systems, the recombinant activin A precursor protein including the African clawed frog-derived prodomain, the human activin A obtained by enzyme-cleaving the recombinant activin A precursor protein including the African clawed frog-derived prodomain, the recombinant activin A precursor protein including the green sea turtle-derived prodomain, the human activin A obtained by enzyme-cleaving the recombinant activin A precursor protein including the green sea turtle-derived prodomain, and the recombinant activin A precursor protein including the zebrafish-derived prodomain.

FIG. 5 shows, in order from the left, the activin A produced by R&D systems, the recombinant activin A precursor protein including the zebrafish-derived prodomain, the recombinant activin A precursor protein including the channel catfish-derived prodomain, and the recombinant activin A precursor protein including the coelacanth-derived prodomain.

From these results, it was revealed that all of the recombinant activin A precursor protein including the zebrafish-derived prodomain, the recombinant activin A precursor protein including the African clawed frog-derived prodomain, the recombinant activin A precursor protein including the green sea turtle-derived prodomain, the recombinant activin A precursor protein including the channel catfish-derived prodomain, and the recombinant activin A precursor protein including the coelacanth-derived prodomain had activity.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to obtain an active protein without refolding even with a protein that is difficult to be folded such as activin. The method for producing human activin A is expected to be utilized for the industrial production of human activin A.

In addition, the recombinant activin A precursor protein of has human activin A activity unlike the conventional findings, and is therefore expected to be utilized in various fields. 

What is claimed is:
 1. A recombinant activin A precursor protein comprising: a human mature activin A sequence; and a non-human activin A prodomain, wherein the recombinant activin A precursor protein does not naturally occur.
 2. The recombinant activin A precursor protein according to claim 1, wherein the non-human activin A prodomain is naturally derived from an amphibian, a reptile, or a fish.
 3. The recombinant activin A precursor protein according to claim 1, wherein the non-human activin A prodomain is naturally derived from a fish.
 4. The recombinant activin A precursor protein according to claim 3, wherein the non-human activin A prodomain comprises a sequence having 90% or more identity with a sequence shown in SEQ ID NO:
 1. 5. The recombinant activin A precursor protein according to claim 4, wherein the sequence having 90% or more identity with the sequence shown in SEQ ID NO: 1 is located at positions corresponding to positions 187 to 196 of a sequence shown in SEQ ID NO:
 2. 6. The recombinant activin A precursor protein according to claim 1, wherein the non-human activin A prodomain has 90% or more identity with a sequence represented by SEQ ID NO:
 2. 7. The recombinant activin A precursor protein according to claim 1, wherein the non-human activin A prodomain has 90% or more identity with a sequence shown in SEQ ID NO:
 3. 8. The recombinant activin A precursor protein according to claim 1, wherein the non-human activin A prodomain has 90% or more identity with a sequence shown in SEQ ID NO:
 4. 9. The recombinant activin A precursor protein according to claim 1, wherein the non-human activin A prodomain has 90% or more identity with a sequence shown in SEQ ID NO:
 5. 10. The recombinant activin A precursor protein according to claim 1, wherein the non-human activin A prodomain has 90% or more identity with a sequence shown in SEQ ID NO:
 6. 11. The recombinant activin A precursor protein according to claim 1, comprising a sequence specifically cleavable by a protease between the human mature activin A sequence and the non-human activin A prodomain.
 12. A polynucleotide encoding the recombinant activin A precursor protein according to claim
 1. 13. A recombinant vector comprising the polynucleotide according to claim
 12. 14. A transformant comprising a polynucleotide encoding the recombinant activin A precursor protein according to claim 1; or a recombinant vector comprising the polynucleotide.
 15. A method for producing the recombinant activin A precursor protein according to claim 1, comprising a step of, in a plant cell into which a polynucleotide encoding the recombinant activin A precursor protein according to claim 1 or a recombinant vector comprising the polynucleotide has been introduced, or in a plant comprising the plant cell, expressing the polynucleotide.
 16. A method for producing human activin A comprising the steps of: in a plant cell into which a polynucleotide encoding the recombinant activin A precursor protein according to claim 11 or a recombinant vector comprising the polynucleotide has been introduced, or in a plant comprising the plant cell, expressing the polynucleotide to generate the recombinant activin A precursor protein; and treating the recombinant activin A precursor protein with the protease. 